1. Field of the Invention
The present invention relates to a low noise amplifier, and more particularly to an improved low noise amplifier.
2. Description of the Prior Art
FIG. 1 is a circuit diagram of a conventional low noise amplifier, which includes input/output matching circuits 10 and 14 and an active device 12.
The noise figure of the low noise amplifier shown in FIG. 1 is expressed in the following relationship: ##EQU1## where NF.sub.min represents a minimum noise figure, R.sub.n represents a normalized noise resistance, .GAMMA..sub.opt represents an optimum noise matching source reflectivity, and .GAMMA..sub.s represents a source reflectivity, respectively. These figures are known as noise parameters and may be determined experimentally. Also, NF is a function of .GAMMA..sub.s. In order to obtain NFrnin, .GAMMA..sub.s and rpt should be equal to .GAMMA..sub.opt. This matching procedure is called a noise matching.
Next, let us consider the power gain of the microwave. There are several gain definitions in the microwave amplifier. Considering a finite S.sub.12 of an active device 12 which is quite useful in the microwave frequencies the useful power gain concept for the design of the input match network of the microwave amplifiers is the available power gain G.sub.A, which is the ratio of the power available from the source to the power available from the network. This is given by ##EQU2## where G.sub.A is not a function of the load reflectivity .GAMMA..sub.L but a function of .GAMMA..sub.s and S parameter of the active device 12. Thus, the process for obtaining the maximum .GAMMA..sub.s and G.sub.A is called an input power matching.
An output matching circuit 14 of the low noise amplifier can be devised using an operative power gain concept defined by the following equations: ##EQU3## where G.sub.p is not a function of .GAMMA..sub.s but a function of .GAMMA..sub.L and S parameters of the active device 12. Thus, the process for obtaining the maximum .GAMMA..sub.L and G.sub.p is called an output power matching, to which a general matching technique may be adopted.
Now, let us consider the stability of the microwave amplifiers. The necessary and sufficient conditions for unconditional stability are given in the following equations: ##EQU4## where .DELTA.=S.sub.21 S.sub.22 -S.sub.12 S.sub.21.
When a stability factor K of the active device is bigger than 1 the input/output power matching can be obtained but when a stability factor K of the active device is smaller than 1, we cannot have an indefinite matching. This is because the power matching points are placed at an unstable area, which is very usual for the microwave amplifiers. Therefore, the stability procedure is highly required for the power matching. A partially stable or unstable active device can be stabilized by using a loading or feedback technique in an input (or output) stage.
However, an additive stabilizer can considerably reduce the noise performance. Thus, in designing a microwave low noise amplifier, a stabilizing circuit should be carefully selected to avoid undesired addition of noise which may be caused by adding the stabilizer.
In designing the low noise amplifier with the common source single gate electric field effect transistor (or common gate bipolar junction transistor), it is well known that the noise matching for accomplishing NF.sub.min is caused by the voltage standing wave ratio (VSWR) or vice versa. This is because the optimum noise matching source reflectivity .GAMMA..sub.opt is quite different from the maximum available power gain matching source reflectivity G.sub.max. Therefore, if the noise matching is performed, the input power matching is not achieved, or vice versa. Thus, it is required to compromise factors among NF, power gain and input VSWR.
However, if .GAMMA..sub.opt and G.sub.max are able to be matched, then NF.sub.min, the maximum available power gain and low input VSWR are simultaneously achieved. This is called a noise and input power simultaneous matching.